Method of manufacturing cathodes for electron tubes



Jan. 6, 1942.

A. FELSNER METHOD OF MANUFACTURING CATHODES FOR ELECTRON TUBES Filed March 29, 1940 Fig. 3

/n ventor. lrizzr fi/sner Patented Jan. 6,1942

METHOD OF MANUFACTURING CATHODES FOB ELECTRON TUBES Artur Felsner, Berlin, Germany, assignor to O. Lorena Aktlengesellschaft, Berlin-Tempelhof, Germany, a company Application In 4 Claims.

This invention relates to the manufacture of cathodes employed in electron tubes and having as large a surface as possible. Diiferent methods for accomplishing this are known. Such cathodes are in general small tubes or tubes of larger size, as may be necessary, which areof a high melting material, such as tantalum or tungsten sheet, and are heated to the emission temperature either directly, that is, by an electric current passing through the tubes, or indirectly, that is, by means of heating bodies located therein. Small tubes of sheet tungsten are difllcult March 29, 1940, Serial No. 326,602 Germany March 9, 1939 In this way, a solid tube is produced which has the layer C intimately united with coil A since to manufacture, and in the case of thoriated tungsten, that is, tungsten having an addition of thorium oxide or so-called thoria, the manufacture appears to be impossible, attempts to subiectrthoriated tungsten to the well known drawing process and thereby to produce a tubular cathod body having failed. The use of thoriated tungsten, however, is desirable because this material aids very much in diminishing the resistance which the electrons when leaving the cathode have to overcome.

The invention has for its object to facilitate the manufacture of thoriated tungsten cathodes and consists in certain features of novelty which will appear from the following description, reference being bad to the accompanying drawing, in which Figs. 1 and 2 are sectional views referred to in explaining the new method, while Figs. 3 to "I are sectional views which show different forms of cathodes manufactured as provided by the invention.

In the arrangement illustrated in Fig. 1 a coil A of thin thoriated tungsten wire is wound around an auxiliary wire 13 serving as a core. This coil, whose turns are in close adjacency to each other, is coated with a thin layer C of tungsten powder or thoriated tungsten powder, which may be mixed with a readily evaporating binding agent and applied to the coil by a spraying or immersing process or in any other suitable manner. Wire B may be of iron or molybdenum, for instance. The assembly A, B, C is calcined for some time in hydrogen gas or some other atmosphere neutral with respect-to. tungsten, whereby the fine-grained powder applied to the coil A is shrunk or sintered thereon. The

wire core B is then removed from the coated coil A by means of a chemical solvent. The tube so obtained is heated for some time at a temperature which is 200 or 300 C. below the melting temperature of tungsten, being highly sintered thereby.

the amorphous tungsten or thoriated tungsten powder constituting this layer is by the intense sintering caused to turn into a crystalline structure. The thorium oxide. being confined in the crystalline structure of the thoriated tungsten wire. evaporates to a very small extent only so that there will remain in'the wire A enough thorium oxide for. the operation of the cathode.

In the case of I cathodes made of unalloyed tungsten the coil A may be omitted and the tungsten powder be applied to wire B directly so as to form a coating C on it, as will appear from Fig. 2.

In order to effect the increase of size which the grains of the tungsten powder undergo the arrangements shown in Figs. 1 and 2 may be made of a powder of tungstic acid mixed with a binding agent. In the case of thoriated tungsten cathodes the tungstic acid is further mixed with a thorium nitrate. During the subsequent reduction, obtained by means of hydrogen, for instance, suitable temperatures are employed to aflect the size of the tungsten grains in such manner that the intense sintering then effected causes a fine crystalline structure to be obtained, whose surface will be smooth as far as possible. In this way, the heat radiation of the cathode is reduced, the energy consumption by the cathode thus being low. I

when manufacturing tubes comprising the aforesaid cathodes the following arrangements may be adopted.

1. Semiindirectly heated cathodes As will be understoodv from Fig. 3 a tungsten wire I is fastened within a tube 2, manufactured as provided by the invention. The wire is held out of direct contact with the tube. For this purpose a short coil 5 of tungsten wire may be wound around the wire 1 at the upper end thereof and may be arranged 'to fill the interspace between wire I and tube 2. In order that the wire I and tube 2 make good electric contact with each other the narrow interspaces between the turns of coil 5 and'between this coil and tube 2 are filled with a paste 4 of tungsten powder. The cathode so obtained is mounted in the respective discharge vessel, whereupon the paste 4 is hardened or sintered by the high temperature to which the discharge vessel is subjected in a well known manner. Wire I may thus be used for supporting the cathode and may also serve as return conductor. In operation the temperature of wire I is higher than the cathode temperature.

Alternatively, the sintering process by which the wire I and tube 2 are secured together may be eflected by arc-welding, preferably the socalled arcatomic welding.

The lower end of the tube 2 may be provided with a ring I made of tantalum or tungsten and fitted with legs 8, or only one such leg. These legs serve as current leads and may also be used for mounting the cathode. They may either be fixed to ring 'I, as by welding, for instance, or may be formed integral therewith. The electric contact between ring I and tube 2 is likewise effected by tungsten powder employed as a paste and hardened by sintering. In the case of largersized devices the ring should be welded to the tube.

As shown in Fig. 4 the tungsten wire I may be arranged to extend beyond the upper end of the tube 2 and may be held in position by insulating material 6, this arrangement being intended especially for electron tubes used in ultra short wave systems.

2. Indirectly heated cathodes In this case, as shown in Fig, 5, the heating wire I is arranged out of contact with the tungsten tube 2. Alternatively, a number of such heating wires may be provided, or a wire or several wires may be arranged in loop-form or in a zigzag, or a wire coil may be employed in well known manner.

Furthermore, a cathode may be located within the tube 2 and this tube itself may be arranged to act as an anode, being heated by electron bombardment from such cathode.

In order that the heat be equally distributed over the entire surface of the cathode the wire core B, Fig. 6, is reduced in diameter at both ends in order to obtain a tubular cathode 2' tapered at its ends. In this cathode the heating wire I may be arranged in the manner represented in Fig. 3 or 5. In operation the cathode so shaped will not be cold at its ends.

Another arrangement for ensuring a uniform distribution of heat is represented in Fig. 7. This shows the ordinary cylindrical tube 2 and a heating wire I' therein which has coil-shaped parts 8 arranged to intensely heat the ends of the tubular cathode 2, which will hence be heated uni formly throughout its length.

What is claimed is:

1. A method of manufacturing cathodes for electron tubes, which consists in winding a coil of wire consisting primarily of tungsten around an auxiliary wire, the turns of the coil so produced being in close adjacency to each other, then coating said coil with a powder consisting primarily of tungsten, hardening this powder by sintering it, and finally removing said auxiliary wire by means of a chemical solvent.

2. A method of manufacturing cathodes for electron tubes, which consists in winding a coil of thoriated wire around an auxiliary wire, the turns of the coil so produced being in close adjacency to each other, then coating said coil with a powder composed of tungstic acid and thorium nitrate, thereupon treating this powder with a chemical reducing agent, then hardening said powder by sintering it, and finally removing said auxiliary wire by means of a chemical solvent.

3. A method of manufacturing cathodes for electron tubes, which comprises winding a coil of tungsten wire around an auxiliary wire, the turns of the coil so produced being in close adjacency to each other, then coating said coil with a powder consisting primarily of tungsten, hardening this powder by sintering, and finally removing said auxiliary wire by means of a chemical solvent.

4. A method of manufacturing cathodes for electron tubes, which comprises winding a coil of thoriated tungsten wire and then coating said coil with a tungsten powder containing thoria.

ARTUR FELSNER. 

